Electron lens utilizing superconductive coils for an electron microscope or the like



SHINJIRO KATAGIRI TAL March 110, 1970 500,269

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Filed June 9. 1957 I FIG. 2 PRIOR T- FIG. I PRIOR ART 7' PRIOR A FIG.

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United States Patent 3 500,269 ELECTRON LENS UTILIZING SUPERCONDUCTIVE COILS FOR AN ELECTRON MICROSCOPE OR THE LIKE Shinjiro Katagiri, Katsuta-shi, Susumu Ozasa, Kodalrashi, Hirokazu Kimura, Koganei-shi, and Toshio Doi and Hiroshi Kimura, Tokyo-to, Japan, asslgnors to Hitachi, Ltd., Tokyo-to, Japan Filed June 9, 1967, Ser. No. 644,845 Claims priority, application Japan, June 10, 1966, 41/37,083 Int. Cl. H0lf 7/00, 7/22, 1/00 U.S. Cl. 335210 12 Claims ABSTRACT OF THE DISCLOSURE An electron lens having a construction in which plural numbers of superconductive coils are used, and at least one of said coils is made to provide a persistent current condition, and further, the current flowing in the other coils is controlled, thereby varying the focal length of said lens.

BACKGROUND OF THE INVENTION This invention relates to an electron lens for an electron microscope or the like and, more particularly to a variable focal length type of electron lens utilizing super conductive coils.

In the magnetic type electron lens used for electron microscopes or the like, the strength of the excitation should be increased in proportion to the square root of the accelerating voltage of the electron if the same focal length is to be obtained as will be obtainable when the accelerating voltage of the electron is raised. For example, assuming that the excitation ampere turns are 1 for 100 kv. accelerating voltage. At this ratio, the ampere turns should be increased to 2.6 times for 500 kv. accelerating voltage, or 4.25 times for 1000 kv. accelerating voltage. And, in this respect, the greater the excitation ampere turns the greater will be the power consumption as well as consumption of the iron and other materials used for the lens construction. In such cases superconductive coils are advantageous for use with the lens. For example, by the use of superconductive coils consisting of niobium, tin or a niobium-zirconium alloy or the like, the capacity of the lens and the power consumption can be minimized and, at the same time, a very strong magnetomotive force can be obtained. Particularly, a stable magnetic field produced by the persistent current of superconductive coils is optimum for the magnetic field for an electron lens. However, the conventional lens using superconductive coils at persistent current conditions has the disadvantage that the focal length of said lens cannot be varied.

BRIEF DESCRIPTION OF THE INVENTION The present invention has as its object the provision of a lens arrangement by which the focal length of the electron lens using plural numbers of superconductive coils may be easily varied.

Another object of this invention is to increase the stability of the lens by utilizing the persistent current condition of a superconductive coil.

Another object of this invention is to provide an electron lens of short focal length even in the case of an electron beam of high accelerating voltage by utilizing. a superconductive coil and thereby obtaining a strong ampere turn.

A further object of this invention is to provide an electron le-ns which is of small size and consumes less power.

In order to achieve said objects, the electron lens of 'ice this invention is provided at least with a number of superconductive coils which are coaxially positioned closely to each other, a part of said coils facing the electron beam axis being magnetically non-shielded, means for maintaining said coils in a super low temperature condition, means for short-circuiting at least one of said coils to establish a persistent current condition and still further, means for adjusting the current of any one of the other coils except said shor-t-circui-ted coil to vary the focal length of said lens.

These and other objects, features and advantages of the present invention will become more apparent from the following detailed description thereof when taken with the accompanying drawings, which illustrate several embodiments of the invention, and wherein:

FIGURES 1 and 7 are diagrams showing conventional electron lens arrangements in which superconductive coils are used;

FIGURE 2 is a diagram showing the distribution of magnetic field strength along the axis A-A of the lenses illustrated in FIGURES 1 and 7;

FIGURE 3 is a diagram showing an embodiment of the electron lens of the present invention;

FIGURE 4 is a diagram showing the distribution of magnetic field strength along the axis A-A of the lens of FIGURE 3; and

FIGURES 5, 6 and 8 are sectional diagrams showing other embodiments of the electron lens of the persent invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIGURE 1, the lens includes a superconductive coil 1 having an iron cover 2. The coil and the iron cover are immersed in a super low temperature liquid, such as liquid helium, and, as a result is reduced to a superconductive condition. A so-called superconductive switch 3 is provided for shor-t-circuiting or opening both ends b and c of said coil. For example, the ends b and c of the coil 1 may be short-circuited by a superconductive wire 4. A heater 5 is wound on the superconductive wire 4 and is connected in series circuit with a power source 7 via a switch 6 and a resistor '10. In this case, the superconductive Wire 4 is immersed in the super low temperature liquid, similarly as the lens component. Through this arrangement and when the superconductive wire 4 is heated by the heater 5, it is released from its superconductive condition and effectively becomes an insulator with respect to the superconductive coil whereby said ends b and c of coil 1 are opened. The superconductive wire 4 is of high resistance at the normal temperature, but the resistance will become nearly zero in the superconductive condition. When heating of the superconductive wire is stopped, the wire 4 will become superconductive once again and the ends b and 0 will be short-circuited. Thus, the superconductive switch 3 is provided to selectively efrec-t connection and disconnection of points b and c.

The points b and c are also connected by a switch 9 in series with a power source 8 and an adjustable resistance. When the switch 9 is closed under the condition where the switch 3 is open, a certain specific current is supplied from the power source 8 to the coil 1 via the adjustable resistance 10. At the instant when the coil 1 reaches a steady state condition, the switch 9 is opened and the switch 3 is closed thus short-circuiting the ends a and b of the coil. At this point, the coil '1 becomes a closed loop circuit, and, at the same time, the coil 1 is kept in the superconductive condition. Therefore, the current flowing therein is everlastingly persistent. This condition is called a persistent current condition.

In this case the distribution of magnetic field produced on the pole 2' is represented, as shown in FIGURE 2, by a magnetic field strength H and an expansion of the distance Z from the axis of said pole. In FIGURE 2, a is a value equal to of the width of Z at which the magnetic field strength H is made /2 of its maximum value H In this relationship the smaller the value of a of the greater the value of H the shorter will be the focal length of the lens. The width a is determined by the diameter d of the pole 2' and also by the. distance h. Hence, even if the ampere turn of the coil 1 is changed, a cannot be changed but only the maximum value H of magnetic field strength H can be changed. In the conventional lens having the structure as in FIGURE 1, the focal length cannot be changed because the ampere turns are invariable once the coil has been reduced to a persistent current condition, as above described.

Referring now to FIGURE 7, which illustrates another prior art arrangement, the coil 1 is divided into two parts; one part serves as a main coil 11 and the other part serves as an auxiliary coil 12. The main coil 11 is reduced to the persistent current condition in the same. manner as the coil in FIGURE 1. The switch 9' is closed and the current supplied from the power source 13 is adjusted by means of a resistor 14. And further, the ampere turns of the auxiliary coil 12 is appropriately changed. By doing so, it is contemplated that the magnetic field strength is to be changed. In this case, however, the auxiliary coil 12 is completely enclosed by the iron cover 2 and all the flux produced from the auxiliary coil 12 is interlinked with the main coil 11. Therefore, a current is induced in the main coil 11, which forms a closed loop circuit, by the act of said flux. The flux produced by this induction current acts to cancel the flux of said auxiliary coil. Consequently, the magnetic field strength H is neither changed nor is, needless to say, a changed. In other words, the focal length cannot be changed, in the manner as heretofore described, and this disadvantage becomes serious when said lens is used for an electron microscope or for similar purposes. For example, in the case of electron microscope wherein the electron beam is focused on the object plane, said focal length will vary when the viewing field is changed or the accelerating voltage is changed even slightly. As a result of this, the focal length should be readjusted. Practically, however, with the known lens arrangement it is impossible to perform a readjustment of the focal length.

The conventional lens having the structure as heretofore described has a grave disadvantage due to the fact that all the flux produced at the auxiliary coil is interlinked with the main coil and, as a result, the focal length cannot be changed once the main coil has been reduced to a persistent current condition. In consideration of this disadvantage, the present invention has its principle in the structure where at least the part of said coils facing the electron beam axis is magnetically non-shielded thereby preventing the flux produced at the auxiliary coil to interlink with the main coil.

More particularly in FIGURE 3, the main coil 11 and auxiliary coil 12 are coaxially positioned to be closed to each other. The vicinity around said coils 11 and 12 is not covered by a magnetic coil cover, but rather only a tub 15 containing liquid helium 16 surrounds the coils which are maintained in a superconductive condition. It is to be noted that each embodiment of this invention has a switching arrangement represented by elements having the numerals 3 through 13 inclusive in FIGURE 3, but such arrangement is not actually shown in the respective FIGURES 3, 5, 6 and 8. Since the coil used for this embodiment is of the air-core type, the distribution of magnetic field does not have such a concentrated shape at the pole, in connection with the embodiment of FIGURE 1, as shown in FIGURE 2. However, because a superconductive coil is used therein, a suflicient magnetomotive force can be obtained even if said coil is small in size. This is Well applicable for practical use.

When current flows in the main coil 11 and auxiliary coil 12, the distributions of magnetic field of the main coil 11 and the auxiliary coil 12 are represented respectively by the curve a and B in FIGURE 4. The combined magnetic field of the two is represented by the curve 7. When only the main coil is reduced to a persistent current condition and the current of the auxiliary coil is changed, the flux thus changed is not all interlinked with the main coil. And, according to the change in the flux, the distribution of magnetic field formed by the auxiliary coil, other than that interlinked with the main coil, is represented by the numeral 2 in FIGURE 4. Thus, the combined distribution of magnetic field is represented by 3. In this case, the maximum magnetic strength H of the field is almost unchanged, but the width a is essentially changed. This tendency becomes remarkable particularly in the case where the auxiliary coil is thin in comparison with the main coil. The focal length can be changed by changing not only the strength of the magnetic field but also by changing the width a. Through adjustment based on said changes, the focal length can be effectively changed.

FIGURE 5 shows another embodiment of the invention, in which the main coil and the auxiliary coil are covered by an iron cover. However, a part of said coils facing the axis A-A is magnetically non-shielded. In this arrangement the magnetic field is concentrated on the axis and, therefore, efficiency is high in comparison with that of the embodiment of FIGURE 3. In this case, however, it is necessary that at least a part of the flux produced at the auxiliary coil 12 be prevented from interlinking with the main coil 11. For this purpose, the two coils are not completely covered.

Further, for the purpose of improving concentration of the flux, an arrangement is necessary, as shown in FIG- URE 6, in which the end of the coil is also covered by the iron cover and, at the same time, inthe portion where the two coils come together, the part facing the electron beam axis A-A' is so arranged that it is magnetically non-shielded by the use of, for instance, a nonmagnetic body 17.

In the embodiments heretofore described, the auxiliary coil was located only on one side of the main coil. However, the auxiliary coils may be located on both sides of the main coil, as shown in FIGURE 8, and by so arranging, the distribution of magnetic field is adjusted. Also, by reversing the polarity of the auxiliary coil with respect to the main coil, the distribution of magnetic field may be adjusted. In the hitherto described embodiments of this invention, the cover used is of ferromagnetic material. In addition, superconductive material may be used for the same purpose whereby its anti-ferromagnetic characteristics are utilized, as indicated in FIG. 8. In addition, iron-cobalt alloy (Fe-Co), which displays stronger ferromagnetic characteristics than that of iron, may also be utilized for said purpose.

As heretofore described, the main coil of the present invention is operated under the persistent current condition and, as a result, current stability is excellent and stability of the magnetic field (AH/H) is better than l0 /min. Current stability of the auxiliary coil is allowable to ten times that of the main coil under the condition where, for example, the ampere turns of the auxiliary coil are made of that of the main coil. Generally, the stability (AH/H) of the lens used for an electron microscope is required to be better than 10* /min. It is obvious that the stability in the persistent current condition can be well above said requirement. The stability of the auxiliary coil is about 10- /min., which can also be good enough for practical application. It is to be noted that part of the flux produced at the auxiliary coil is to contribute. to the adjustment of the distribution of magnetic field. Therefore the current stability will become more temperate than 10 /min.

In the embodiments having been described, the winding radius of the main coil and the auxiliary coil are equal. But different radii may of course be adapted therefor.

By use of this invention, strong excitation ampere turns can be obtained; e.g. 20000 ampere turns at about 1 cm. coil section, without using an electromagnet which requires a considerable amount of power, as previously de scribed. Also, an electron lens of short focal length can be obtained even with an electron beam of 1000 kv. accelerating voltage, similarly as in the case of 100 kv.

We, have shown and described several embodiments in accordance with the present invention. It is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art and we, therefore, do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

We claim:

1. An electron lens arrangement for an electron microscope or the like comprising:

a plurality of annular electrical coils coaxially positioned in close relationship, at least a portion of the inner periphery of each of said coils facing the axis thereof being magnetically non-shielded, and said coils being magnetically coupled with each other through said magnetically non-shielded portions to produce a composite magnetic field,

superconductive means substantially surrounding said coils for maintaining said coils at a super low temperature in a superconductive state,

first means for selectively establishing a current flowing in said coils, and

second means for short circuiting one of said coils to establish a persistent current condition therein,

said first means including third means for adjusting the current flowing in any of said coils other than said one short circuited coil for adjusting said composite magnetic field and thereby the focal length of said lens.

2. The combination defined in claim 1 wherein the entire inner periphery of said coils facing the common axis thereof is magnetically non-shielded.

3. The combination defined in claim 1 wherein said entire coils are magnetically non-shielded.

4. The combination defined in claim 1 wherein the plural annular coils are axially stacked to be a composite annular coil structure and further including a cover of ferromagnetic material covering the outer periphery and both top and bottom surfaces of the annular coil structure with the exception of the inner periphery thereof.

5. The combination defined in claim 4 wherein said cover is made of iron.

6. The combination defined in claim 4 wherein said cover is made of an iron-cobalt alloy.

7. The combination defined in claim 1, wherein said plural annular coils are axially stacked to be in a composite annular coil structure and the entire surface of said composite annular coil structure with the exception of at least an annular band area on the inner periphery of said annular coil being covered with a ferromagnetic material.

8. The combination defined in claim 7 wherein said annular band area is covered with a non-ferromagnetic material.

9. The combination defined in claim 1, wherein said plural annular coils are axially stacked to be in a composite annular coil structure and the entire surface of said composite annular coil structure with the exception of at least an annular band area on the inner periphery of each annular coil being covered with a superconducting material having anti-ferromagnetic properties.

10. The combination defined in claim 7, wherein each pair of coils mutually abutting to each other have their axially non-shielded annular band areas at their mutually adjacent end parts on the inner periphery thereof.

11. An electron lens arrangement for an electron microscope or the like comprising:

a main annular electrical superconductive coil with at least an annular hand area on the inner periphery thereof being magnetically non-shielded;

an auxiliary annular superconductive coil coaxially aligned with said main coil, at least an annular hand area on the inner periphery thereof being magnetically non-shielded so that the auxiliary coil is magnetically coupled with the main coil through the annular areas thereof to produce a composite magnetic field;

superconductive means for maintaining said main and auxiliary coils at a super low temperature to render the coils in a superconductive state, respectively;

first means for establishing a current flowing in said coil;

second means for selectively short-circuiting said main coil to establish a persistent current condition therein; and

third means for supplying a variable current to said auxiliary coil to adjust the composite magnetic field, thereby adjusting the focal length of the electron lense arrangement.

12. The combination defined in claim 11, wherein the annular band areas are provided at the mutually abutting end parts of the main and auxiliary coils.

References Cited UNITED STATES PATENTS 3,008,044 11/1961 Buchhold 335--21O XR 3,351,754 11/1967 Dietriech et al. 335-210 XR GEORGE HARRIS, Primary Examiner US. Cl. X.R. 

